Importance of Physics in Science
Introduction to Physics
Physics is the study of the basic laws that govern matter. Physics covers the science of the very large, and includes the study of bodies in motion, forces, heat and energy, waves, electricity, magnetism and atomic physics.
Waves in Physics
A wave is a phenomenon whereby energy is moved without the transference of any material. X – rays, ultraviolet rays, light and radio waves all travel at the same speed through a vacuum. Examples of waves include water waves, sound waves, light and X – rays. There are two main types of waves : transverse and longitudinal. In transverse waves the vibrations are perpendicular to the direction of travel, whereas in longitudinal waves the vibra tions are parallel to the direction of travel.
Wavelength and Frequency
The distance between successive wave crests is called the wavelength, X ( lamda ). The frequency ( f ) of a wave is defined as the number of complete oscillations per second. Frequency is measured in hertz ( Hz ). Audible sound frequencies range from 20 Hz ( a low rumble ) to about 20,000 Hz ( a shrill whistle ). The speed of a sound wave in air at 20°C ( 68°F ) is 344 m / s, but in water sound travels at 1461 m / s and in steel its speed is 5000 m / s.
Properties of Waves
- Reflection : Reflection is the process whereby part or all of a wave is returned when it encounters the boundary between two different materials or media. Important example of a wave reflection is an echo, when sound waves bounce off a faraway surface.
- Refraction : Refraction is the change of direction of a wavefront as it passes obliquely ( at any angle which is not perpendicular or parallel ) from one medium to another in which its speed is altered. An example is when light enters a lens or prism – the light is bent. It is the principle of refraction that makes the lenses in spectacles work.
- Diffraction : Diffraction occurs when waves passing through a slit which is narrow compared to the wavelength are spread out and depart from the expected straight line direction. This explains how we can hear the words of someone who is facing away from us.
- Interference : Interference is the phenomenon that occurs when two or more waves combine together as dictated by the principle of superposition. The superposition principle states that when two waves are in the same place at the same time, their amplitudes ( heights ) are combined. If the resultant wave amplitude is greater than that of the individual waves then constructive interference is taking place. If the resultant wave is smaller, then destructive interference is taking place.
Electromagnetic waves are caused by a mutual fluctuation in electric and magnetic fields. All the properties of sound and water waves, such as refraction and diffraction, exist in electromagnetic waves, but they differ in that they are able to transmit energy in a vacuum. They travel extremely fast : at 299,792,458 m / s in a vacuum. Electromagnetic waves include light, microwaves, infrared radiation and X – rays. The electromagnetic spectrum is the collective set of waves over a broad range of wavelengths, from gamma waves ( wavelength 10 – 16 m ) to radio waves ( wavelength 103 m ).
The Kinetic Theory of Matter
When a red hot piece of iron cools down, it transfers energy to its surroundings in three possible ways: conduction, convection or radiation.
- Conduction : Heat conduction occurs when kinetic and molecular energy pass from one molecule to another. Metals are good heat conductors because of electrons that transport energy through the material.
- Convection : Heat convection results from the motion of the heated substance. Convection is the main mechanism for mixing the atmosphere and diluting pollutants emitted into the air.
- Radiation : All bodies radiate energy in the form of electromagnetic waves. Radiation may pass across a vacuum, and thus the Earth receives energy radiated from the Sun. A body remains at a constant temperature when it both radiates and receives energy at the same rate.
Physics Four Fundamental Forces
The interactions between matter can be explained by four forces:
- Gravitational : The weakest of the four forces, the gravitational force is the mutual attraction between masses. Although its effect is small in the realm of subatomic particles, it has great cosmic power, and is the force that holds solar systems and galaxies together.
- Electromagnetic : This force explains the magnetic field and the electron-nucleus structure of an atom.
- Strong : Some 100 times stronger than the electromagnetic force, the strong force holds together the protons and neutrons within an atomic nucleus.
- Weak : This force is associated with the radioactive beta-decay of some nuclei. The electromagnetic and weak forces have recently been shown to be part of an electro – weak force.
Physics States of Matter
- Gases : Gases are readily compressible by a factor up to one thousand, showing that there must be large spaces between the molecules. The molecules in a gas are able to translate ( move freely ), rotate and vibrate. The temperature of a gas is a measure of the average kinetic energy of its molecules.
- Liquids : Liquids are not easily compressible. Liquids are much more dense than the corresponding gases from which they are condensed. In a liquid the molecules are in contact with each other, yet able to move around as the molecules vibrate and disturb each other.
- Solids : Solids are not at all easily compressible. In a solid the particles vibrate ever more vigorously as the temperature is raised.
Special and General Theories of Relativity
Special theory of relativity was formulated by Einstein.
The special theory of relativity states that nothing can exceed the speed of light, which is the same in all inertial ( nonaccelerating ) time frames and that all inertial time frames are equally good for carrying out experiments. Special relativity equates space with time and matter with energy. In special relativity time and space have to be considered as unified and not as two separate things; this merged entity is known as space-time. Matter and energy are aspects of the same phenomenon, matter energy, and are linked in the famous equation: E = mc2 where E is the energy released when mass ( m ) is destroyed, as in a nuclear reaction, and c is the velocity of light. It indicates that when 1 kg is destroyed, 9 x 1016 joules of energy are released.
In his general theory of relativity, Einstein considered systems accelerating with respect to each other. He concluded that the effects of acceleration and gravity were equivalent. Using the theory, he was able to explain a discrepancy in the slow rotation of the planet Mercury’s elongated orbit, which Newtonian mechanics had been unable to explain. Einstein explained the effect of a large concentrated mass, in this case the Sun, by saying it ‘warped’ the space around it. Thus space, or rather spacetime, is curved. The degree of curvature depends upon the magnitude of the mass, and so matter-energy determines the curvature of space – time.
Physics Heisenberg Uncertainty Principle
The Heisenberg Uncertainty Principle ( 1927 ) concerns the accuracy with which the position and momentum of a particle ( such as an electron ) can be determined. Thus an accurate determination ( measurement ) of one quantity leads to the impossibility of measuring the other accurately. As both quantities cannot be known precisely, it follows that limits are set on the accuracy of any predictions made using these measurements.
String Theory in Physics
One of the problems with current models of physics is that relativity and quantum theory are, to some extent, inconsistent with each other. So far, no physicist has been able to provide a quantum theory of gravity which would combine the two fields. This search for a Grand Unified Theory ( GUT ) has occupied many scientists for much of this century. A recent possible theoretical model is string theory. String theory suggests that the universe is made up of very small vibrating strings. These strings are about 100 billion billion times smaller than a proton, which itself is less than a billionth of a metre in size. In addition, these strings are vibrating in ten dimensions.
The different resonances at which these strings vibrate produce the varied fundamental particles, such as quarks, leptons and electrons, that are known to exist. String theory also predicts the relativistic equations of Einstein. Thus the theory has managed to unify several separate aspects of modern physics. However, it too is not yet complete : there are many millions of mathematical solutions to the field theory of strings, and the correct solution is still beyond grasp of contemporary physics.
Radioactivity in Physics
Radioactivity was gradually understood in terms of the disintegration of atomic nuclei. The three types of radiation are alpha decay, beta decay and gamma decay. Alpha radiation occurs when an unstable nucleus breaks down so as to eject a fast-moving nucleus of helium, which consists of two protons and two neutrons. Beta radiation occurs when an unstable nucleus breaks down so that a neutron in the nucleus splits into a proton, which stays in the nucleus, and an electron ( or positron, which is identical to an electron but with a positive charge ) is ejected from the nucleus at very high velocity. After alpha or beta decay, the remaining nucleus is very often left in an ‘excited’ state from which it ‘relaxes’ with the emission of a gamma ( or X – ray ) photon. This is the origin of gamma radiation.
Nuclear Fission and Fusion in Physics
Otto Hahn and Fritz Strassman ( 1902 – 1980) discovered nuclear fission in 1938. When an isotope of uranium – 235 was bombarded with neutrons, it split into two lighter nuclei along with, on average, three neutrons. These neutrons were capable of bombarding and splitting other nuclei, causing more fission to take place. If the mass of uranium – 235 was above a certain level ( the critical mass ) this produced a chain reaction. It was the production of this chain reaction which, in turn, led to the development of the first nuclear bomb. Fission is used in both nuclear reactors and atomic weapons. Nuclear fusion occurs when two small nuclei collide and combine, breaking the weak nuclear force and releasing energy. This type of reaction releases considerably more energy than a fission, process for a given mass of material. However, unlike nuclear fission, humankind has not yet found a way to properly contain or control the process. Many scientists today are searching for the key to controlled room – temperature fusion referred to as ‘cold fusion’. An example of uncontrolled fusion reaction is the hydrogen ( thermonuclear ) bomb, which relies on the fusion of light atoms to give heavier atoms, with the destruction of matter releasing the observed energy.
Physics Nuclear Particles
The proton and neutron, which were once thought to be the basic blocks of matter, are now known to be made up of over 200 elementary particles. Elementary particles can be divided into two types: hadrons which are heavy particles subject to the strong force, and leptons which are small particles not subject to the strong force. Elementary particles have a further distinction between fermions, which are permanently existing particles, and bosons, which can be produced and destroyed freely.
Every type of particle is thought to have a companion antiparticle, which is opposite to it in some characteristic way. For instance, the positron, with positive charge, is the antiparticle of the electron, with a negative charge. Some particles, such as the photon, serve as their own antiparticles. Protons and neutrons are composed of simpler particles named quarks. The six types ( ‘flavours’ ) of quark are : up, down, charmed, strange, top and bottom. The proton is considered to consist of two up quarks and a down quark, whilst the neutron consists of two down quarks and an up quark. Mesons are short – lived subatomic particles composed of two quarks each. Mesons jump between protons and neutrons, thus holding them together. Neutrinos are particles which can carry much energy away from nuclear reactions, such as those involved in radioactivity, but they are difficult to detect, as they only interact very weakly with ordinary matter. They are capable of passing right through the Earth undetected.
Motion and Mechanics
Kinematics covers a broad range of topics, from bodies falling to earth, to the description of bodies moving in a straight line, to circular motion.
- Speed : Speed is the ratio of a distance covered by a body in a given amount of time, to that time. It is measured in metres per second.
- Velocity : velocity is speed measured in a particular direction. Velocity is a vector quantity, which is one in which both the magnitude and direction are stated.
Acceleration and Deceleration :
Acceleration is the rate of change of velocity. Acceleration may be defined as the change in velocity over a given time interval. Acceleration is measured in m / s2 ( or ms – 2 ).
Physics Newton’s Laws of Motion
Newton’s three laws of motion state the fundamental relationships between the acceleration of a body and the forces acting on it.
- A body will remain stationary or travelling at a constantvelocity unless it is acted upon by an external force : Newton’s first law explains why we lurch forward in a car when it suddenly breaks, and why we are pushed back into our seats when a car suddenly accelerates.
- The resultant force exerted on a body is directly proportional to the acceleration produced by the force :The second law of motion can be expressed in an equation: force = mass x acceleration, or F = ma. Forces are measured in newtons. A force of 1 newton will accelerate a mass of 1 kg by 1 m / s2.
- To every action there is an equal and opposite reaction : When a bullet is fired, thegun recoils backwards. This is caused by a reaction force on the gun from the bullet. From this law can be derived the principle of the conservation of momentum. Momentum, which Newton called the ‘quantity of motion’, is the product of mass and velocity.
Newton’s Law of Gravitation
According to this law every particle in the universe attracts every other particle in the universe. Newton’s Law of Gravitation is thus: F = Gm1m2 / r2, where G is the ‘universal gravitational constant’. Further experiments on gravity proved that : G = 6.67206 x 10 – 11 Nnr – 2kg – 2.
Physics Measurement Units
- Acre : A measure of land, 4,840 square yards ( 4,046 square metres ).
- Ampere : Unit for measuring the strength of an electric current. It is the amount of current sent by one volt through a resistance of one ohm.
- Angstrom : It is the unit for measuring the wavelength of light. It is one hundred-millionth of a centimetre.
- Astronomical unit : A unit of length equal to the mean radius of earth orbit. It is 149,597,870 km ( 92,955,800 miles ).
- Bar : Unit of atmospheric pressure. One bar is equal to a pressure of 106 dynes per sq cm.
- Barrel : For measuring liquids. One barrel is equal to 31.1 / 2 gallons in US and 36 imperial gallons in Britain.
- Bushel : Unit of dry measure for grain, fruit, etc. It is equal to 4 pecks or 8 gallons.
- Calorie : Unit for measuring the amount of heat required to raise the temperature of one gram of water through 1°C. It is used as the unit for measuring the energy produced by food when oxidised in the body.
- Carat : Unit of weight for precious stones and pearls. It is equal to about 3.17 grains troy or .2 of gram. It is also a measure of the purity of gold alloy indicating how many parts out of 24 are pure.
- Coulomb : Unit for measuring the quantity of an electric current. It is the amount of electricity provided by a current of one ampere flowing for one second.
- Decibel : Unit for measuring the volume of sound. It is equal to the logarithm of the ratio of the intensity of sound to the intensity of an arbitrarily chosen standard sound.
- Dyne : Amount of force that causes a mass of one gram to alter its speed by one centimetre per second for each second during which the force acts. This unit of force is in CGS ( metric ) system.
- Erg : Unit of work or energy in CGS ( metric ) system. It is the amount of work done by one dyne acting through a distance of one centimetre.
- Farad : Electromagnetic unit of capacitance. It is equal to the amount that permits the storing of one coulomb of charge for each volt of applied potential difference.
- Fathom : Unit for measuring the depth of water or the length of a rope or cable. One fathom is equal to 6 feet.
- Gallon : It is a measure of liquid.
- Gross : 12 dozens or 144.
- Hertz : Modern unit for measurement of electromagnetic wave frequencies.
- Horse power : Unit for measuring the power of motors or engines. It is equal to a rate of 33,000 foot-pounds per minute, i.e., the force required to raise 33,000 pounds at the rate of one foot per minute. HP = 746 watts.
Physics Heat and Work
Modern physics sees heat as energy collectively possessed by the particles making up a gas, liquid or solid. A body which possesses energy has the ability to do work. Work is done when a force ( F ) moves through a distance ( d ) : W = F x d. If F is measured in newtons and d in metres, then W is measured in Nm, otherwise called joules.
Thermodynamics in Physics
Thermodynamics is the study of the behaviour and properties of heat, energy and temperature within systems.
- The first law of thermodynamics : The first law of thermodynamics states that the total amount of energy in any closed system always remains the same. In other words, energy is always conserved as it is transferred from one form to another.
- The second law of thermodynamics : The second law of thermodynamics states that heat will always flow from a hotter object to a colder one, and not the other way round. It involves the term entropy. Entropy is a measure of the disorder of a system.
- The third law of thermodynamics : The third law states that on approaching absolute zero, extracting energy from a system becomes increasingly harder. All bodies have thermal energy, or heat. Absolute zero is the theoretical point at which a body ceases to have any heat. This value is – 273.15°C ( -459.67°F ) or 0°K ( Kelvin ). At this temperature, which is impossible to physically attain, the molecules in a body will cease to vibrate, and thus the body will have no internal energy.
Electromagnetism is the study of the effects caused by stationary and moving electric charges.
Pieces of some metallic ores, such as lodestone, are magnetic when suspended freely from a thread they point north – south. Such magnetic compasses have been used since 500 BC.
At present, science recognises a spectrum of electromagnetic radiation that extends from about 10-15 m to 10° m.
- Radio waves have a large range of wavelengths, from a few millimetres up to several kilometres.
- Microwaves are radio waves with shorter wavelengths, between 1 mm and 30 cm, and are used in radar and microwave ovens.
- Infrared waves of different wavelengths are radiated by bodies at different temperatures. The Earth and its atmosphere, at a mean temperature of 250 K (-23°C or -9.4°F) radiates infrared waves with wavelengths centred at about 10 micrometres.
- Visible waves have wavelengths of 400 – 700 nanometres (nm; 1 nm = 10 – 6 m).
Ultraviolet waves have wavelengths from about 380 nm down to 60 nm. The radiation from hotter stars, above 25,000°C ( 45,000°F ), shifts towards the violet and ultraviolet parts of the spectrum.
- X-rays have wavelengths from about 10 nm to 10 – 4 nm.
- Gamma rays are emitted by certain radioactive nuclei in the course of nuclear reactions. It is now known that the Earth itself has magnetic properties. An important feature of a magnet is that it has two poles, one of which is attracted to the Earth’s magnetic North Pole, while the other is attracted to the South Pole.
Static Electric Charges
Static electricity involves electric charges at rest. In 1785, Coulomb formulated the Law of Attraction and Repulsion between electrically charged bodies : F= kQ1Q2/r2 where F is the force, k is a constant, Q1 and Q2 are the sizes of the charges ( + or – ), and r is the distance between the charges.
Electric Current in Physics
In the late 1790s, Count Alessandro Volta made the first battery. The first practical primary cell ( non – rechargeable ) was produced by John Frederic Daniell ( 1790 – 1845 ) in 1836, using zinc and copper electrodes. This was followed by the first secondary ( rechargeable ) cell invented by Gaston Plante (1834 – 1889) in 1859. This cell, based upon the use of lead and sulfuric acid, is still employed as the car battery. The Leclanche ‘Dry’ Cell of the 1860s was developed into the modern ordinary torch battery. It has a zinc case and a carbon electrode down its centre.
In 1827, George Simon Ohm introduced the concept of resistance ( to the flow of an electric current ) and stated Ohm’s Law : I = V/R which translates as : I, the current in amperes ( amps ) in the conductor, V, the potential difference in volts across the ends of the conductor and R, the resistance of the conductor.
In 1831, Michael Faraday confirmed his hypothesis that magnetism could produce electricity when he found that a magnet moving in a coil of wire induced a current in the wire when the wire was present in an electric circuit. These discoveries soon led to the electric motor and to the dynamo, a device for generating electricity by spinning a coil of wire between the poles of a suitably shaped magnet.
James Clerk Maxwell predicted in 1873 that by using oscillating electric currents it should be possible to generate electromagnetic waves which travel at the speed of light. In the late 1880s, Heinrich Hertz produced such waves, which were soon used for wireless transmissions.
Measuring current :
1 amp current has 6 x 1023 / 96,500 or approximately 6 x 1018 electrons passing a given point in the circuit every second. A one – volt battery supplies one joule of energy to eachcoulomb of electric charge that it produces. Mechanical power is measured in watts ( joules per second ). If a 6v battery is supplying 5 amps ( 5 coulombs per second ), it is thus producing 6 joules per coulomb and a total of 5 x 6 = 30 joules per second. The general formula for power supplied is : P = V x I where P is the power in watts, V is the voltage ( also known as Potential Difference ) and I is the current in amps.
Atomic Theory in Physics
John Dalton revolutionized science in 1803 when he hypothesized that atoms of different chemical elements, such as hydrogen and oxygen, had different characteristic masses. J.J. Thomson discovered the first subatomic particle – the electron – in 1897. Ernest Rutherford revealed the presence of a dense central nucleus in atoms in 1911. In 1913, Niels Bohr proposed that electrons in atoms are confined to certain allowed ( restricted ) orbits. In 1924, Prince Louis Victor de Broglie produced a better description of atoms, which included the wavelike property of the quickly orbiting electrons. By 1928, Edwin Schrodinger, Wolfgang Pauli and Max Born had described the atom using quantum mechanical principles.
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